A modern computing system typically comprises a central processing unit, a memory unit, and supporting hardware necessary to communicate both within the system and to the outside world. Within the computing system, communication buses may exist to transmit information and instructions between the processing unit and the memory unit. To communicate with the outside world, input and output (“I/O”) devices may be used. Common examples of input devices include the keyboard and mouse. Output devices include printers and display devices such as a monitor. In addition, storage devices such as hard disk drives, floppy disk drives, and optical disk drives may serve as both input and output devices.
In conventional hard disk drives, information in the form of analog or digital data is recorded on concentric tracks on a magnetic disk. The disk is spun at a very high speed while a read/write head moves radially over the surface of the disk in order to read information from or write information to the tracks. Information which is read out from a disk may be supplied to a device known as a Viterbi decoder to generate a reproduction of an original data signal recorded or the disk.
One type of information commonly written to and read from the tracks is servo information. Servo information may comprise control information such as address and position information to allow proper alignment of the read/write head with respect to the tracks, as well as other track identification information. Servo information is often written on a portion of the disk platter known as a servo wedge. Within a servo wedge, the servo information is broken up into multiple servo fields, each typically containing a different type of servo information. While reading such information or performing a seek operation to pinpoint a specific track, a read/write head may radially traverse multiple tracks. When the servo wedges are not properly aligned along the radial direction, “radial incoherence” (RI) may result. RI is the timing offset that occurs when a head moves between radially adjacent tracks in which the servo wedges are not properly aligned. The waveform signal read by the head may undergo a phase shift due to the RI, called the RI phase, which leads to errors in decoding the recorded servo information.
Typically, RI is detected by a RI detector with a fixed or symmetric RI detection threshold in a servo channel. However, a RI detection threshold that is set too high may fail to detect RI when it exists. Conversely, a RI detection threshold that is set too low may detect false positives when no RI exists. In addition, noise or other errors can corrupt a sequence of RI data.
Thus, there is a need for an improved system for RI detection that increases the probability of detecting RI and reduces the bit error rate (BER) of a decoded signal read from a disk.